One of the more difficult problems associated with any borehole is to communicate measured data between one or more locations down a borehole and the surface, or between down-hole locations themselves. For example, communication is desired by the oil industry to retrieve, at the surface, data generated down-hole during operations such as perforating, fracturing, and drill stem or well testing; and during production operations such as reservoir evaluation testing, pressure and temperature monitoring. Communication is also desired to transmit intelligence from the surface to down-hole tools or instruments to effect, control or modify operations or parameters.
Accurate and reliable down-hole communication is particularly important when complex data comprising a set of measurements or instructions is to be communicated, i.e., when more than a single measurement or a simple trigger signal has to be communicated. For the transmission of complex data it is often desirable to communicate encoded digital signals.
Widely considered for borehole communication is to use a direct wire connection between the surface and the down-hole location(s). Communication then can be made via electrical signal through the wire. While much effort has been spent on “wireline” communication, its inherent high telemetry rate is not always needed and very often does not justify its high cost.
Another borehole communication technique that has been explored is the transmission of acoustic waves. Whereas in some cases the pipes and tubing within the well can be used to transmit acoustic waves, commercially available systems utilize the various liquids within a borehole as the transmission medium. Examples of the use of hydraulic lines for downhole power generation and telemetry are described in WO 2004/085796 A1 and WO 2005/024177 A1.
Yet another borehole communication system is based on optical signals. Communication over an optical fiber is accomplished by using an optical transmitter to generate and transmit laser light pulses that are communicated through the optical fiber. Downhole components can be coupled to the optical fiber to enable communication between the downhole components and surface equipment. Examples of such downhole components include sensors, gauges, or other measurement devices.
Typically, an optical fiber is deployed by inserting the optical fiber into a control line, such as a steel control line, that is run along the length of other tubing (e.g., production tubing). The control line is provided as part of a production string that is extended into the wellbore.
As described for example in the published United Kingdom patent application GB 2409871 A, optical fibers can also be applied to intervention, remedial, or investigative tools as being deployed by a wireline, slickline, coiled tubing, or some other type of conveyance structure.
Further uses of optical fibers for communication inside a wellbore are described in the related U.S. Pat. Nos. 5,898,517, 5,808,779 and 5,675,674, which describe an optical fiber modulation and demodulation system using Bragg gratings and piezoelectric crystal combination.
However, a major limitation of conventional optical communications systems applied to hostile environments such as hydrocarbon production wells is the need to terminate the fiber at each node of the communication system. The termination might be accomplished by connecting the optical cable to the communication node, which involves expensive parts and lengthy procedures to ensure that the connection is hermetically sealed against the ingress of the downhole fluids. Alternatively, special optical connectors might be used that are suitable for the hostile environment; however these are expensive. In both cases these connections, whether spliced or connectorised are expensive and create a weak point that could degrade the overall reliability of the communications system.
Outside the technical field of borehole telemetry, Berwick M. and al. describe a magnetometer in their paper: “Alternating-current measurement and non-invasive data ring utilizing the Faraday effect in a closed-loop fiber magnetometer” Optics Letters Vol. 12. No. 4, 1987. Berwick M. and al. also propose to use the system as data ring. Similar methods and apparatus can be found in the U.S. Pat. Nos. 6,462,856 B1 and 4,996,692.
It is therefore an object of the present invention to provide optical fiber based communication system that overcomes the limitations of existing devices to allow the communication of data into one or more nodes along the fiber without breaking into the fiber. The system provided is particularly for hostile environment where the fiber is enclosed in a protective tube or sheath. An example suitable for the invention could be the communication between a down-hole location and a surface location.